Although the basics of radar have changed little since the first edition was published. One of the main changes is in the handling of the MTI (moving target indication) radar (Ch. 4.
THE SIMPLE FORM OF THE RADAR EQUATION
It is a characteristic of the specific target and is a measure of its size as seen by the radar. If the effective area of the receiving antenna is denoted A., the power P received by the radar is.
RADAR BLOCK DIAGRAM ANO OPERATION
The most common form of cathode ray tube display is the plane position indicator, or PPI (Fig. 1.3a), which maps in polar coordinates the location of the target in azimuth and range. In a phased array, the beam is scanned by electronically varying the phase of the currents across the aperture.
RADAR FREQUENCIES
The specific numerical frequency limits should be used when appropriate, but the letter designations in Table 1.1 may be used when a short notation is desired.
RADAR DEVELOPMENT PRIOR TO WORLD WAR II
The NRL work on the detection of aircraft by CW interference was kept classified until 1933, when several Bell Telephone Laboratories engineers reported the detection of aircraft in the course of other experiments.5 The NRL work was disclosed in a patent filed and granted Taylor, Young. , and Hyland6 on a •• Radio Object Detection System." The type of radar described in this patent was a CW interference radar. The development of the magnetron was one of the most important contributions to the realization of microwave radar.
APPLICATIONS OF RADAR
In space, radar has aided in spacecraft guidance and remote sensing of land and sea. Some of the largest ground-based radars are for satellite detection and tracking.
THE RADAR EQUATION
PREDICTION OF RANGE PERFORMANCE
Its specification should include a statement of the probability that the radar will detect a given type of target at a given range. In this chapter, the simple radar equation will be expanded to include most of the important factors affecting radar beam performance.
MINIMUM DETECTABLE SIGNAL
The signal-to-noise ratio required to ensure adequate detection is one of the most important. Van Vleck and Middleton3 showed that maximizing the signal-to-noise ratio at the IF output is equivalent to maximizing the video output.
RECEIVER NOISE
Although the detection decision is usually based on measurements at the video output, it is easier to consider maximizing the signal-to-noise ratio at the output of the IF amplifier than at the video. The noise figure can therefore be interpreted as a measure of the degradation of the signal to noise ratio when the signal passes through the receiver.
Regardless of whether the noise is generated by a thermal mechanism or by some other mechanism, the total noise on the entire output of the receiver can be considered equal to the thermal noise power obtained from an "ideal" receiver multiplied by a factor called the noise figure. The noise NO is measured over the linear part of the receiver's input-output characteristic.
PROBABILITY-DENSITY FUNCTIONS
One of the more useful concepts of probability theory needed to analyze the detection of signals in noise is the probability density function. Because each measurement must yield a certain value, the integral of the probability density over all values of x must equal one; 2.10).
SIGNAL-TO-NOISE RATIO
The probability that the noise voltage envelope will be between the values of V1 and. The average time interval between noise-only threshold crossings is defined as the false alarm time Tra,.
INTEGRATION OF RADAR PULSES
However, many pulses are usually returned from any particular target (111cach radi~r scan and can be used to improve detection. The number of pulses111n) rotate from a point targetJ as the radar antenna scans through its ~~~\~~-~. j-~!!Jt is ' / ts,.\. The integration improvement factor (or the integration loss) is not a sensitive function of either the probability of detection or the probability of false alarm.
RADAR CROSS SECTION OF TARGETS
The radar cross-section of the thin bar (and similar: objects) is small when viewed from the side (0 = 0°), since the physical area is small. The radar cross-section of the T-38 aircraft in frontal attack is shown in Table 2.1.
CROSS-SECTION FLUCTUATIONS
The two probability density functions of Eq. 2.39a) and (2.39b), describing the Swerling fluctuation models, are special cases of the chi-square distribution of degree 2m. 3 8 The probability density function is. The ratio of the variance to the mean value of the cross section is equal to tom-112 for the chi-square distribution.
TRANSMITfER POWER
It is difficult to be precise about which statistical model to use for any given objective. The important parameters affecting the range are the total transmitted energy 11£1 • the transmit gain G, the effective receiving aperture Ae and the receiver noise figure F".
2.IO PULSE REPETITION FREQUENCY AND RANG(£ AMBIGUITIES The pulse repetition frequency (prf) is determined primarily by the maximum range at which
- ANTENNA PARAMETERS
- SYSTEM LOSSES
- PROPAGATlON EFFECTS
- OTHER CONSIDERATIONS
It is not unusual to find no account of carrier loss taken into the radar equation. Some of these other forms of the radar equation are given elsewhere in this text.
THREE
CW AND FREQUENCY-MODULATED RADAR
THE DOPPLER EFFECT
The relative velocity can be written u, = v cos 6, where vis is the target speed and (} is the angle made by the target trajectory and the line joining radar and target. CW radar provides a measurement of relative velocity that uses can be used to distinguish moving targets from stationary objects or debris.
CW RADAR
The doppler sign can be determined on video by other means, as described later.). The received signal and part of the heterodyne transmitter in the detector (mixer) to give a difference signal.
FREQUENCY-l\'IOOULATEO CW RADAR
If the rate of change of the carrier frequency is Jo, then the beat frequency is A block diagram of the FM-CW radar with a sideband superheterodyne receiver is shown in Fig.
Doppler navigation radar measures the vector velocity relative to the reference frame of the antenna assembly. Two rays give the two components of the aircraft velocity tangential to the surface of the earth.
The remote unit at the other end of the line receives the signals from the master unit and amplifies and retransmits them. The phases of the returned signals on the master unit are compared with the phases of the outgoing signals.
FOUR
MTI AND PULSE DOPPLER RADAR
INTRODUCTION
Coherent means that the phase of the transmitted signal is preserved in the reference signal. The phase of the coho is linked to the phase of the transmitted pulse each time a pulse is generated.
DELAY-LINE CANCELERS
The output of the two single-delay-line cancellers in cascade is the square of that from a single canceller. The relative response of the dual-canceler compared to a single-delay-line canceler is sho.yn in fig.
RANGE-GATED DOPPLER FILTERS
The disadvantage of a staggered prf is its inability to eliminate secondary echoes of interference. The selection of the lower limit can be at the operator's choice or it can be done flexibly.
OIGITAL SIGNAL PROCESSING
The phase of the pulse train relative to that of the doppler signal in Fig. Note that the enhancement factor of a two-pulse canceller is almost as good as that of the 8-pulse doppler filter bank.
OTHER MTI DELAY LINES
Each cell of the ground noise map stores the average value of the zero rate filter output for the past eight scans (32 seconds). With each scan, one-eighth of the zero-velocity filter's output is added to seven-eighths of the value stored in the map.
Lll\tllTATIONS TO MTI PERFORMANCE
The calculation of the limitation to the enhancement factor can be found in a manner similar to that of the clutter fluctuations described previously. The parameter C/L is the ratio of the rms clutter power to the receiver IF limit level.
NONCOHERENT MTI
The local oscillator of the non-coherent radar is not as frequency stable as in the coherent MTI. The clutter serves the same function as the reference signal in the coherent MTI.
PULSE DOPPLER RADAR 57
The output from the amplitude detector is followed by an MTI processor such as a delay line equalizer. This is possible if the doppler frequency shift is at least comparable to or greater than the spectral width of the transmitted signal.
MTI FROM A MOVING PLATFORM
One is in the direction of the antenna pointing and shifts the center frequency of the Doppler interference spectrum. In one embodiment, the frequency of the coherent oscillator (coho) is changed to compensate for the shirt in the Doppler interference frequency.
OTHER TYPES OF MTI
Capon, J.: Optimal weighting functions for the detection of sampled signals in noise, IEEE Trans., vol. The Effect of Internal Oscillations and Scanning on Interference Attenuation in MTI Radar, IRE Trans., vol.
FIVE
TRACKING RADAR
TRACKING WITH RADAR
Oh great, it is possible to reconstruct the track of the target from the sampled data. This can be accomplished by providing the PPI scope operator with a pencil to mark the target dots on the front of the scope.
SEQUENTIAL LOBING
The scanning fan-heam search radar can also provide tracking information to determine the target's path and determine its future position. When the voltages in the two switched positions are equal, the target is on axis and.
CONICAL SCAN
The purpose of the phase-sensitive detector is to conveniently provide the sign of the error signal. With AGC, the output of the angular error detector is proportional to the dilTarence signal normalized (divided) by the sum signal.
TARGET-REFLECTION CHARACTERISTICS ANO ANGULAR ACCURACY 30 · 41
The signal reflected from the surface can be thought of as originating from the image of the target reflected from the earth's surface. In this condition, the values of L\/E in the direction of the target and its image are equal.
The normal monopulse radar receiver uses only the fundamental (or out-of-phase) component of the difference signal. The in-phase and quadratic components of the error signal define a complex angkenoi:-sign.al.
ACQUISITION
The conical scan of the SCR-584 was performed during the search mode and was actually a Palmer scan in a helix. As a consequence, the number of hits returned from a target when searching at a constant scan rate depends on the position of the target within the search area.
OTHER TOPICS
Thus, the pointing of the antenna is made open-loop based on the stored target trajectory prediction updated by the radar measurements. There is nothing unique about any of the individual processes that go into on-axis tracking.
For this type of dynamic calibration, the radar must be large enough to track satellites. High-resolution monopulse.66 It was noted earlier in this chapter that the presence of multiple scatterers within the range-resolution cell of the radar leads to scintillation.
They can each be applied individually, if desired, to any tracking radar to improve tracking accuracy.
COMPARISON OF TRACKERS 1
In this section, only the conical scan radar and the amplitude c6mparison monopulse are compared. The tracking accuracy of a monopulse radar is superior to that of the conical scan radar due to the absence of target amplitude fluctuations and due to its greater signal-to-noise ratio.
5.IO TRACKING WITH SURVEILLANCE RADAR
The size of the small gate will be determined by the accuracy of the track. Campbell: Experimental results of the complex indicated angle technique for multipath correction, IEEE Trans., vol.
RADAR TRANSMITTERS
INTRODUCTION
